**Core Concept**
The immediate source of energy for cellular metabolism refers to the primary energy currency of the cell, which is used to drive various cellular processes such as muscle contraction, nerve impulses, and biosynthesis.

**Why the Correct Answer is Right**
The correct answer is ATP (Adenosine Triphosphate). ATP is the primary energy currency of the cell, and it is produced through the process of cellular respiration. During cellular respiration, glucose is broken down to produce ATP, which is then used to drive various cellular processes. The high-energy phosphate bonds in ATP are broken down to release energy, which is then used to drive cellular reactions. The enzyme ATP synthase plays a crucial role in the production of ATP during oxidative phosphorylation.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen is a stored form of glucose, but it is not the immediate source of energy for the cell. While glycogen can be broken down to produce glucose, which is then used to produce ATP, it is not the direct source of energy.
**Option B:** NADH is a coenzyme that plays a crucial role in the electron transport chain, but it is not the immediate source of energy for the cell. NADH is generated during glycolysis and is used to produce ATP during oxidative phosphorylation, but it is not the direct source of energy.
**Option C:** FADH2 is another coenzyme that plays a crucial role in the electron transport chain, but it is not the immediate source of energy for the cell. Like NADH, FADH2 is generated during glycolysis and is used to produce ATP during oxidative phosphorylation, but it is not the direct source of energy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that ATP is the primary energy currency of the cell, and it is produced through the process of cellular respiration. ATP is used to drive various cellular processes, and its breakdown releases energy that is used to drive cellular reactions.

**Correct Answer: D. ATP**